It's as slippery as the contrapuntal chords in a herky-jerky Charlie Parker saxophone run, or the way Miles Davis' career has shifted from bebop to cool jazz to fusion, each time helping turn on a whole new stream of music.

Such tension, inbred in jazz itself, will be found in the contrasting styles of the performers at this June's Hampton Jazz Festival.

"Jazz absorbs elements of other music," said Joseph Lowrey, jazz producer for WHRO-FM (89.5), a public radio station in Norfolk. "It grows out of American culture. Jazz, unlike most other musical forms, is uniquely American."

Today's jazz takes in parts of the blues, soul, rock, doo-wop, Latin music, gospel, Dixieland and straight-ahead pop, to name a few of its influences. It doesn't have to have swing to mean a thing.

"A definition doesn't say much," said Robert Ransom, coordinator of the jazz studies program at Hampton University. "It's a repertoire of music, a way of performing, that allows more freedom and leeway.

"Other than that, a definition of jazz varies with the performer."

New backbeats have evolved from old rhythms. Today's jazz shares in the music's rhapsodic history, helping make all that jazz a wide-arching "all."

Bluesy vocal harmonies from Ella Fitzgerald to Sarah Vaughan, speaking in jazz tongues while scorching through a scat section, have been countered by Bobby McFerrin or Al Jarreau, putting pop-oriented vocal gurgles and hums to wax.

Or consider trumpeter Dizzy Gillespie, blowing out his cheeks like two helium balloons to move notes up and down, ushering in Stanley Jordan, moving fingers up and down the neck of his guitar to play the bass and melody parts.

The divergent players of jazz normally mean more than the music's sum. That's what makes the form such an elusive bird to classify.

"Jazz always has been a music of individuals," said Lowrey, who must sift through hundreds of albums received each year at his radio station. "I resist trying to classify artists. It's all jazz and it's all unique in some way."

FROM THE VERY BEGINNING, jazz didn't fit under one convenient label. It started as a cross-cultural hybrid, taking elements of European harmonics and African rhythms, said Rogers Brown, assistant professor of music with Norfolk State University. With a nod to New Orleans Dixieland and Delta blues, the music took seed in the early part of this century.

In the 1930s and '40s, the music of jazz mainly meant swing. It was good-time dance music, characterized by consummate entertainer Louis Armstrong waving a handkerchief and wearing a wide grin, and big-band show stoppers Benny Goodman and Glenn Miller swaying the crowd with "In the Mood."

Even then, there was discord. Duke Ellington was dabbling in classical structures with suites such as "Mood Indigo." And big-band crooners like Frank Sinatra and Rosemary Clooney were making the music even more accessible to the masses.

But then the dance music died, especially after World War II. In came bebop, a faster-tempoed, all-over-the-chordal-spectrum form of jazz that negated dancing. The short, nervous notes played by its practitioners - notably Gillespie, pianist Thelonious Monk and Parker - turned as many people away from jazz as it attracted.

"It required more attention to understand it," Ransom said of bebop, which grew in popularity in the early 1950s. "It was as different as it could be from pop music, from dance music. People couldn't tap their toes, so they had to sit at their tables and just listen to it."

In the early 1950s, bebop begat "cool" or "West Coast" jazz, a more laid-back approach that stretched each note instead of playing it quick and staccato. Its adherents included drummer Gerry Mulligan and the Dave Brubeck Quartet, along with the pioneering work of Miles Davis.

But while cool jazz mellowed the bebop, free or avant-garde jazz in the late 1950s took another seemingly irrational step. In some free jazz, melody was hardly distinguishable at all; at times the music sounded more like an instrument-tuning session. It was punctuated by the improvised saxophone compositions of Coltrane and Ornette Coleman.

Then came the mix of jazz and classical music, played by the Modern Jazz Quartet in the early 1960s on classical instruments such as the French horn and the oboe. More sounds were brought in from over the fence of the strict jazz environs.

An "East Coast" jazz style, also guided in formation by Davis with Horace Silver and Art Blakey, meant the founding of fusion in the late 1960s. Fusion merged rock and rhythm-and-blues elements with bebop jazz.